Creep strain equations of Grade 92 steel which is used in boilers and piping systems of ultra-supercritical (USC) thermal power plants were developed based on the results of creep tests on smooth round bar specimens of three kinds of Grade 92 steels. In these equations, primary creep behavior was represented by a power-law and tertiary creep behavior was described by an exponential function. Creep parameters were determined as a function of creep rupture times which were calculated from stress and absolute temperature. Additionally, generalized creep failure criteria considering the multiaxial stress were established on the basis of results of creep tests on circumferentially notched round bar specimens. These creep strain equations and creep failure criteria were incorporated into finite element analysis software. Then, creep failure analyses were carried out and the resulting deformation behavior and rupture times were compared with the experimental results. Creep rupture lives were predicted with a good accuracy, within a factor of two in most cases.
In this study, to clarify the behaviour of micro- and macro-creep damage progression for P92 under multiaxial stress field, interrupted creep tests, analysis of multiaxial stress and detailed the cross-sectional observations were conducted on a circular notched round bar specimen which produces the multiaxial stress field due to the plastic constraint. As a result, creep voids were initiated at the early stage and they were formed up to the final fracture. These phenomena were found to be detected using direct current potential drop (DCPD) method. These results concern the development of the measurement of creep crack initiation. The distribution of high void area fraction was in good agreement with that of high hydrostatic stress and high multiaxial stress. This result indicates that multiaxial stress affects the void formation. Furthermore, the micro-creep damage of each interrupted specimen was evaluated by using the electron backscatter diffraction (EBSD) method which can analyse crystallographic misorientation caused by creep strain. The results of EBSD analyses indicated that the value of grain reference orientation deviation (GROD) closely concerns the void initiation.
The method of computer aided vacancy migration analysis has been developed to predict the stress induced voiding (SiV) reliability. In this method, distribution of hydrostatic stress was calculated by the finite element analysis (FEA), and vacancy concentration distribution was calculated by the finite difference analysis (FDA). In this paper, SiV acceleration tests were conducted in various widths of Cu lines in organic ultralow-k dielectric (Cu/Ta/ULK/SiCN) or silicon oxide dielectric (Cu/Ta/SiO 2 /SiCN) and these results were compared with the results of the vacancy migration analyses. The number of SiV failures increased in the line with width of 0.35 m for Cu/Ta/SiO 2 /SiCN interconnects and 0.20 m for Cu/Ta/ULK/SiCN interconnects, respectively, under SiV acceleration tests. The results obtained by vacancy migration analysis show similar behavior as the results of SiV acceleration tests. These results reveal that the vacancy migration analysis is useful to predict reliability of interconnects. #
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